1. Field of the Invention
The present invention relates to a process for preparing a flocculant for water treatment, and more particularly to a simple and low-cost process for preparing a flocculant for water treatment having high treatment capability.
2. Description of the Prior Art
Purification of irrigation water or wastewater to remove suspended substances and other impurities is made by such a water-treatment process as pouring a flocculant into irrigation water or wastewater to coagulate and deposit the impurities for removal. The flocculants used for this purpose include aluminum sulfate, polyaluminum chloride and ferric chloride.
Of the flocculants, aluminum sulfate and polyaluminum chloride are widely used. However, because aluminum is an amphoteric metal, the aluminum causes a problem in that it remains in treated water as soluble aluminum coordinating with an organic substance in the water, e.g., an organic acid produced by algae. Also, aluminum flocculants cannot achieve as high coagulation performance at low water temperatures and, therefore, need to be poured in excess.
In order to solve the above problems, attentions are now drawn to metal-silica inorganic polymer flocculants which are prepared by adding a water-soluble metal salt, such as an iron salt, to polymerized silicate, in particular an iron-silica inorganic polymer flocculant in which the metal salt is an iron salt, because of their high and stable coagulation performance and nontoxicity.
Such flocculants are disclosed in, for instance, JP-B-4(1992)/75796 and Japanese Patent No. 2,732,067, which are obtained by adding a silicate aqueous solution to such an inorganic acid aqueous solution as of hydrochloric acid or sulfuric acid in a container, e.g., a beaker, to prepare a silica sol having an SiO2 concentration of about 1 to 6%, then aging the silicate aqueous solution at around room temperature for several hours with stirring to proceed polymerization, and adding such a metal salt aqueous solution as of an iron salt to the solution.
The metal-silica inorganic polymer flocculants produced by the above process realize high coagulation performance and have a long gel time, maintaining its flocculating properties in spite of a long-term storage. Also, they exhibit high coagulation performance even at low water temperatures and provide many other advantages as a water treatment agent.
However, the silica sol produced according to the above method by adding a silicate aqueous solution to an inorganic acid aqueous solution cannot have an SiO2 concentration higher than 70 g/L (a little under about 7%), because partial non-uniformity occurs extremely easily in a mixture of the silicate aqueous solution and the inorganic acid aqueous solution and the non-uniform portions start immediately gelling to make it impossible to obtain a uniform silica sol in an attempt of high-concentration silica sol preparation. Accordingly, the concentration of SiO2 has to be set low in order to produce the flocculants having good flocculating properties, which lowers productivity.
Moreover, to improve the flocculating properties of the metal-silica inorganic polymer flocculants and to realize practical coagulation performance, the silica sol needs to be aged while being heated to about 60° C. at the time of aging (thereby the polymerization of silicate proceeds and the molecular weight of silicate increases). Therefore, a heating apparatus is required and costs for industrial production are accordingly raised.
This is the reason why the metal-silica inorganic polymer flocculants have not been practically used in industries in spite of their various advantages mentioned above.
Therefore, a process has been desired which can produce a metal-silica inorganic polymer flocculant exhibiting excellent coagulation performance, without involving any specific apparatuses, at low cost and in large quantities.
In order to solve the above problems, the present inventors made an earnest study focusing on the process of preparing a uniform and high-concentration silica sol disclosed in JP-B-4(1992)/54619. As the result of their additional study, there have been revealed that a silica sol, which is prepared by an applied process derived from the above publication, and aged to have a specific viscosity, is useful as a raw material to produce the metal-silica inorganic polymer flocculant.
The present inventors further studied on the viscosity of the silica sol, the SiO2 concentration in the silica sol and flocculating properties of the resulting flocculant and, as a result, completed the present invention.